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  #21  
Old 03-02-2017, 12:35 PM
jrs14855 jrs14855 is offline
 
Join Date: Apr 2008
Location: Lake Havasu City AZ
Posts: 2,005
Default Testing

Mark Frederick was operating in uncharted territory. By his own admission he was operating at a higher speed than ever before in that airplane.
The statement required at Reno regarding testing is only as good as the integrity of the operator.
The testing itself, if done to Reno protocol, is rather meaningless. Dive testing could be performed 100 times successfully and the next time something flutters. This has proven to be the case with early T18's and Stephens Acro derivatives among others. The ONLY way to know for sure is a full professional analisis, WAY beyond the budget of most Sport Class racers.
Trim tabs are a completely separate issue. If a trim tab or tab linkage fails at race speed, statistically it is fatal, whether an RV, Rocket or composite airplane.
So I stand by my statement that the Reno racers, SARL and others are operating in uncharted territory.
I have an airplane that I built with the intention of racing in SARL. However, in that category, the bar has been raised to a speed well in excess of the speed I am willing to operate my airplane at.
Another completely separate issue is turbulence penetration speed. A Pitts or Pitts derivative is capable of surviving severe turbulence at race speed. Not so with some of the Sport Racers because of the lower stall speed, and in some cases lower G limit.
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  #22  
Old 03-02-2017, 08:25 PM
F1Boss's Avatar
F1Boss F1Boss is offline
 
Join Date: Aug 2006
Location: Taylor Texas
Posts: 625
Default For the record:

The Reno Sport Class test speed requirement is a dive to 105% of the expected fastest lap speed. My expected lap speed the year I had a trim tab let go was 265MPH. The plane was tested to 285mph. So, I exceeded the Sport Class requirements in several dive tests. I know the plane had been to 250+KTAS many times against a 240KTAS Vne.

As Bob remarked, the drop off speed agreed by the racers and briefed by the Pace Pilot and Observer in that section was 160KIAS if I recall correctly. It turned out that we were dropped at ~200KIAS. The NTSB noted that the failure occurred at 306mph TAS.

I'm sorry to be mixing KTS and MPH, but that is what the report reads.

I'll guess the temp at that time was about 80F - so you can see we were very close to 265MPH at the drop - in a dive - at full power. Not good, as it turned out. I ended up 20MPH past where I knew I was safe - QUICKLY. Again I am guessing, but I think the failure at 306MPH occurred about 15 SECONDS after the drop off at 265MPH.

Again, the failure of the tab was caused by an assembly mistake - not a structural overload. But: I installed the part, so it is my fault.

I would not call this particular event a flutter event in the same manner that airframe flutter is being discussed in this thread. I will suggest that you all keep an eye on your trim tabs tho!

Since having correctly built parts installed, that ship has been tested to far more than 306MPH - several times, with no issues. But the lap speeds remain in the 260MPH range. Oh well.

Also, the Sport Class drop-off speeds are monitored quite closely due to this event, and are probably +/- 2KIAS of what is agreed to, and briefed. That is a very good thing, as we have all seen what can happen if the drop speed is not what the pilots think it is.

Carry on!
Mark
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  #23  
Old 03-02-2017, 09:09 PM
RV8JD's Avatar
RV8JD RV8JD is offline
 
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Default

Quote:
Originally Posted by F1Boss View Post
I would not call this particular event a flutter event in the same manner that airframe flutter is being discussed in this thread. I will suggest that you all keep an eye on your trim tabs tho!

Mark
Mark,

I'm not too familiar with your incident. What would you call it? (Note that elevator/elevator tab flutter is airframe flutter.)
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  #24  
Old 03-03-2017, 04:31 PM
jrs14855 jrs14855 is offline
 
Join Date: Apr 2008
Location: Lake Havasu City AZ
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Default Reno

Some more food for thought:
Miller Gem flew into a dust devil at Reno and emerged in many pieces.
The homebuilt Mustang clone with Lear wing, structural failure of vertical tail during race.
GP5, wing failure during qualifying. A number of people knew that one was just a question of when.
So far no takers on my question of maneuvering/turbulence penetration speed in races. The various RV's appear to be considerably more vulnerable in this regard than the Rockets.
I am not opposed to the Reno Sport Class Racers. I am concerned that some are not concerned with the issue of an encounter with severe turbulence at a speed far in excess of maneuvering speed.
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  #25  
Old 03-06-2017, 08:58 PM
Dave Anders Dave Anders is offline
 
Join Date: Feb 2007
Location: cottonwood, calif
Posts: 18
Default

i have always been concerned about flutter with my rv-4. the fastest i ever went with the stock rv-4 aft fuselage was about the speed at the triaviathon competition 250.7 MPH in 1997. after that i modified the fuselage with the fast back and an significantly different type of elevator and rudder construction.
the first surface to flutter in the rv-4 style tail aircraft is most likely the rudder/vertical stabilzer. there have been aircraft in the past that fortunately landed after experiencing sever empanage flutter. one aircraft i am referring to even had the aft portion on the turtledeck in front of the empanage crush inward as if you twisted a cola can in your hands. the bulhead between the longerons under the empanage was torn corner to corner into 4 pieces which gives you an idea of the movement of the tail. the empanage surfaces were trashed and the rudder was inoperable. the pilot did a good job getting it down in 1 piece and told me he was being careful while testing Vne.
my fast back is epoxy bonded and revited. my rudder and elevators have epoxy bonded and revited trailing edges that are about .060" thick with a square trailing edge. they are mass balanced including the rudder. they have .020" skins. they are stronger and stiffer than original construction. the rudder has ribs in every other stiffener location instead of just stiffeners. the trim tab uses close tolerance fits (actually reamed) so as to not allow any play. these are things you can find in any aero engineering book on flutter resistant construction.
i have been able to talk with richard vangrunsven and ken kreuger over the years and both of them understood what i was doing, didn't want me to get hurt, and just suggested ideas that might help. neither endorsed my efforts.
i have been told that the deeper aft fuselage as in my fast back was a good modification to increase torsional stiffness. i am told however, unless you have had flutter testing done on your aircraft you don't know where it may be. also that, since we all build our own they may not be exactly comparable.
this is the third set of control surfaces i have put on my aircraft over it life. the first 2 failed from cracking. i also completely broke the vertical stablizer spar into from side to side about 3" below the top hinge before it was counter weighted. i consider myself a pretty good builder and i know the spar didn't break because of poor construction. i also never self anything unusual during any flight.
the efforts of all racers are to be as careful as they can be, while being aware that the plane is being operated experimentally, perhaps beyond it's design.
reno has it's special safety requirements in an attempt to protect the pilots and fans. bob mentioned the safety changes regarding starting procedures for the metal planes that are of the rv style and modifications thereof. those planes now run on a smaller course so they are less likely to be able to get to questionable speeds which could be dangerous.
the fastest i have gone in a race situation is just over 264 MPH TAS but i don't want anyone to think that would be safe to duplicate. that was straight level and smooth.
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  #26  
Old 03-07-2017, 06:03 AM
Weasel's Avatar
Weasel Weasel is offline
 
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Quote:
Originally Posted by Dave Anders View Post
i have always been concerned about flutter with my rv-4. the fastest i ever went with the stock rv-4 aft fuselage was about the speed at the triaviathon competition 250.7 MPH in 1997. after that i modified the fuselage with the fast back and an significantly different type of elevator and rudder construction.
the first surface to flutter in the rv-4 style tail aircraft is most likely the rudder/vertical stabilzer. there have been aircraft in the past that fortunately landed after experiencing sever empanage flutter. one aircraft i am referring to even had the aft portion on the turtledeck in front of the empanage crush inward as if you twisted a cola can in your hands. the bulhead between the longerons under the empanage was torn corner to corner into 4 pieces which gives you an idea of the movement of the tail. the empanage surfaces were trashed and the rudder was inoperable. the pilot did a good job getting it down in 1 piece and told me he was being careful while testing Vne.
my fast back is epoxy bonded and revited. my rudder and elevators have epoxy bonded and revited trailing edges that are about .060" thick with a square trailing edge. they are mass balanced including the rudder. they have .020" skins. they are stronger and stiffer than original construction. the rudder has ribs in every other stiffener location instead of just stiffeners. the trim tab uses close tolerance fits (actually reamed) so as to not allow any play. these are things you can find in any aero engineering book on flutter resistant construction.
i have been able to talk with richard vangrunsven and ken kreuger over the years and both of them understood what i was doing, didn't want me to get hurt, and just suggested ideas that might help. neither endorsed my efforts.
i have been told that the deeper aft fuselage as in my fast back was a good modification to increase torsional stiffness. i am told however, unless you have had flutter testing done on your aircraft you don't know where it may be. also that, since we all build our own they may not be exactly comparable.
this is the third set of control surfaces i have put on my aircraft over it life. the first 2 failed from cracking. i also completely broke the vertical stablizer spar into from side to side about 3" below the top hinge before it was counter weighted. i consider myself a pretty good builder and i know the spar didn't break because of poor construction. i also never self anything unusual during any flight.
the efforts of all racers are to be as careful as they can be, while being aware that the plane is being operated experimentally, perhaps beyond it's design.
reno has it's special safety requirements in an attempt to protect the pilots and fans. bob mentioned the safety changes regarding starting procedures for the metal planes that are of the rv style and modifications thereof. those planes now run on a smaller course so they are less likely to be able to get to questionable speeds which could be dangerous.
the fastest i have gone in a race situation is just over 264 MPH TAS but i don't want anyone to think that would be safe to duplicate. that was straight level and smooth.
Thanks for all the details Dave! I for one have had quite a few questions about your evolving project and you answered several here.
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